Admins would rarely replies because if they give users taht authority to pm them then there will be hundread of thousands of pm's in there inbox which is hard to reply .
Buying account is always a risk ..?
You might be contact to admin here: https://bitcointalk.org/index.php?board=24.0
If you have any problems related to accounts. Post your problems there. Admin will answer your question if he is around or in a while.
I don't think that's a good idea.
1) Everybody who reads the thread will know your alt.
2) Sellers won't disclose username before you trade.
3) Admins don't involve in these type of things.
Best way is to use escrow and tell him/her to check post history and don't spam. Keep your post quality high.